Generally, discrete-time radio frequency (RF) is a newly emerging field in wireless digital communications wherein analog continuous-time RF signals that are transmitted over-the-air are directly sampled into a discrete-time sample stream suitable for digital signal processing. A typical wireless digital communications device would use analog filters, duplexers, mixers, analog-to-digital converters (ADC), etc. to convert the analog continuous-time RF signals into a digital data stream that is suitable for digital signal processing. Unfortunately, analog circuit components, especially components such as capacitors, inductors, resistors, etc. necessary for the analog filters are difficult to integrate into an integrated circuit. This is especially true for the precise values of these components required for use in filters. Of course, it is the desire of the manufacturer to maximize the degree of integration for the wireless transceivers. This is because the more highly integrated a wireless transceiver can become, the lower the production costs for the transceiver and the transceiver will typically use less power during operation.
Discrete-time RF involves the direct conversion of the analog continuous-time RF signal into discrete-time sample stream through the use of a direct sampling mixer, without having to undergo any intermediate analog continuous-time filtering, downconversion, etc. An example of a direct RF sampling mixer is one that uses current to perform its sampling. The current-mode direct sampling mixer converts the received analog continuous-time RF signal into a current that is then integrated by a sampling capacitor. The charge on the sampling capacitor is then periodically read to produce the discrete-time sample stream.
The analog continuous-time RF signal being directly converted into a discrete-time sample stream may often contain more than a desired signal located in a frequency band of interest (commonly referred to as a signal of interest). In many circumstances, there are interferers along with the signal of interest being sampled by the direct RF sampling mixer. The interferers may be the result of noise sources, such as other radio frequency devices and communications networks operating in close proximity with the direct RF sampling mixer, large electrical motors, electrical appliances, etc. The interferers may be located relatively far away from the signal, close to the signal (commonly referred to collectively as out-of-band interferers), or they may actually occur at frequencies that also carry the signal (commonly referred to as in-band interferers).
In the case when the interferers are in-band, active interferer detection and cancellation may be an only option for removing the interferers. However, when the interferers are out-of-band, filtering can be used to eliminate the interferers.
Filtering can be used to eliminate interferers that are out-of-band, and can take place either in an analog domain or a digital domain. Analog filtering occurs early on, perhaps as early as immediately after the analog RF signal is received by an antenna. Digital filtering, on the other hand, can only occur after the discrete-time sample stream created from the analog RF signal has been converted into a digital data stream. This implies that any digital filtering that is to take place, must occur later in the signal processing sequence.
A problem that is associated with out-of-band interferers is that, while they may have no direct impact on the signal of interest, they may be significantly larger in magnitude than the signal of interest. If this is the case, then it is required that certain RF front-end electronics, such as amplifiers, have good linearity. Linearity is required so that the presence of the large interferers do not distort the performance of the RF front-end electronics in such a way that the electronics do not operate properly on the relatively smaller signal magnitudes of the signal of interest. If the out-of-band interferers are eliminated, then the linearity of the RF front-end electronics can be relaxed due to a reduction of the overall dynamic range of the signal being provided to the electronics, e.g., only the dynamic range of the signal of interest must be dealt with by the front-end electronics.
The direct conversion of the analog continuous-time RF signal into a discrete-time sample stream by the direct RF sampling mixer can include a built-in finite impulse response (FIR) filtering operation. The FIR filtering comes as a result of an accumulation and decimation of multiple samplings of the analog RF signal into a single discrete-time sample by a sampling capacitor. However, the direct RF sampling of the analog RF signal using fixed current gains and constant capacitive loads may result in only FIR filter with constant coefficients. In many occasions, it is desired that arbitrary-coefficient filtering be available to help eliminate interferers, anti-aliasing, etc. Additional filtering can be added, but only with the expense of additional hardware.
One disadvantage of the prior art is that the use of analog filters to eliminate out-of-band interferers can entail the use of high-order analog filters if the out-of-band interferers are close to the signal of interest. High-order analog filters can be difficult to implement, especially on an integrated circuit.
A second disadvantage of the use of analog filters to eliminated out-of-band interferers is that while low-order analog filters can be realized relatively easily, but they are likely to not be able to remove the close-in interferers, therefore, the requirement of good linearity in the RF front-end electronics must be maintained.
A disadvantage of constant-coefficient FIR filtering is that the filtering may have high sidelobes and an insufficient roll-off rate to help eliminate aliasing and/or interferers. Additional filtering can be added to perform these needed tasks, but constant-coefficient filters with low cut-off frequencies are difficult to realize in integrated circuits.